Thin film technology typically employs vacuum deposition, sputtering or chemical vapor deposition (CVD) in a vacuum chamber to deposit a conductive or resistive film upon a substrate. Thin film deposition thickness ranges from about 100 to 5,000 angstroms.
Thick film technology differs from thin film technology in many ways. Typically, thick film technology combines a conductor or semiconductor, glass and a screening agent to form a resistive or conductive paint. The mixture is screened or dipped onto a substrate and fired at a temperature higher than the melting point of the glass to bond the mixture to the substrate.
The screening agent is typically burned off during the firing process. Noble metal thick film resistors are typically fired in air. Base metal thick film resistors are typically fired in an inert atmosphere in order to control oxidation during the firing process.
Thick film resistor paint thickness ranges from about 10 to 40 microns, or about 100 times thicker than thin film resistors.
Thick film resistor (TCR) has been typically adjusted by varying the chemical composition of the resistor prior to firing. Many factors influence the resulting change in (TCR), including at least in part: chemical properties, consistency of blend, impurities, and the thickness of the applied resistor paint. Some additional factors include firing time and temperature, out-gassing and oxidation of the resistor paint during firing, and the number of firings. The combustion products, produced during firing, can have a detrimental effect on the properties of the resistor, if they are not removed.
The (TCR) of a specific thick film resistor composition is very sensitive to firing time and temperature.
Thick film resistors are often fired in an oven having several temperature control chambers, with the firing time in each chamber dependent upon chamber size and the speed of the belt passing through the oven, upon which the thick film resistor is placed. These ovens are sometimes equipped with a means to limit the oxygen content within at least a portion of the oven to inhibit oxidation of the resistive paint during firing.
It has not been practical to monitor the (TCR) of a thick film resistor network during firing in such ovens on a commercial basis, nor to adjust the firing time or temperature in response to the change in (TCR) of the resistor during firing.
Laser trimming has been used to cut away a portion of the resistive material to obtain a desired resistor geometry, but laser trimming does not greatly affect the (TCR) of the remaining resistor network.
Thermal trimming of thin film resistors is known, as disclosed in U.S. Pat. No. 3,420,706, and in an article published in Thin Solid Films, 125 (1985) pages 53 56 entitled: "Effect of Annealing on the Electrical Resistance of Thin Films of Alkaline Earth Metals" by P. Renucci, L. Gaudart, J. P. Petrakian and D. Roux.
An article by C. W. White and P. S. Percy, entitled: "Laser and Electron Beam processing of Materials," page 28, published by the Academic Press in 1980 provides useful background information.
Also noted is an article by A. Gat and J. F. Gibbons, "A Laser-Scanning Apparatus for Annealing of ion Implantation Damage in Semiconductors," Applied Physics Letters 32, No. 3 (1978).